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@PHDTHESIS{Mester:190118,
      author       = {Mester, Achim},
      title        = {{Q}uantitative {T}wo-{L}ayer {I}nversion and {C}ustomizable
                      {S}ensor-{A}rray {I}nstrument for {E}lectromagnetic
                      {I}nduction based {S}oil {C}onductivity {E}stimation},
      volume       = {249},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2015-03058},
      isbn         = {978-3-95806-035-7},
      series       = {Schriften des Forschungszentrums Jülich Reihe Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {viii, 119 S.},
      year         = {2015},
      note         = {RWTH Aachen, Diss., 2014},
      abstract     = {Electromagnetic (EM) measurement methods oer the great
                      potential to non-invasively and contactlessly obtain
                      geological and hydrological soil properties of the uppermost
                      six meters of the subsurface with an areal resolution in the
                      sub-meter range. The presented work is focused on
                      small-sized frequency domain `electromagnetic induction'
                      (EMI) systems which combine the transmitter (Tx) and
                      receiver (Rx) unit in one portable construction and obtain
                      the apparent electrical conductivity ($\sigma_{a}$) of the
                      sensed soil volume by inducing electrical currents and
                      measuring the responding electromagnetic field. The sensing
                      depth of EMI instruments depends on the sensor conguration
                      and in particular the coil orientation and Tx-Rx separation.
                      In principle, multi-conguration EMI data can be inverted for
                      the electrical conductivity distribution over depth.
                      However, there is a demand for efficient inversion
                      algorithms and high-quality EMI data from different sensing
                      depths to perform such an inversion. Here, a novel
                      one-dimensional global-local inversion approach is
                      implemented which evaluates the mist between EMI data and
                      forward modeled data for a two-layer soil using a L1-norm
                      objective function. The global approach is based on a grid
                      search for reasonable model parameters in combination with
                      the local-sensitivity forward model. The two soil models
                      with the smallest misfit are refined using the (local)
                      simplex search algorithm with the more precise full solution
                      electromagnetic forward model. The algorithm is analyzed
                      using synthetic EMI data. Applying the inversion on
                      quantitative EMI transect data from two commercial devices
                      with eight different sensor configurations results in a
                      two-layer electrical conductivity model with lateral and
                      vertical conductivity changes that are in good agreement
                      with a collocated electrical resistivity tomography data
                      set. To improve the depth-resolution beyond available fixed
                      congurations, a novel EMI prototype system (ElMa1) with
                      customizable sensor-array is developed, containing multiple
                      modular sensor units which can be flexibly arranged by the
                      operator for each survey, ensuring optimal depth-sensitivity
                      (i.e. coil orientations and Tx-Rx separations) for the
                      specific investigation. The sensor units consist of
                      coil-based transmitter and receiver circuits which allow for
                      the measurement of the magnetic flux and the sensor
                      impedance in a frequency range between 3 and 33 kHz,
                      respectively. To allow for flexible sensor congurations,
                      data processing and signal optimization, the transmitter
                      current and the receiver voltages are separately digitized
                      using 24-bit analog-to-digital converters (ADC's) which
                      provide a high dynamic range and phase stability. For a
                      measurement time of 0.5 s, the ElMa1 system achieves an
                      instrumental $\sigma_{a}$-accuracy of 1 mS/m at 20 kHz for
                      the intended Tx-Rx separation of 1.0 m and an accuracy of 10
                      mS/m for a less favorable conguration with smaller Tx-Rx
                      separation of 0.3 m and smaller measurement frequency of 5
                      kHz, both observed under stable temperature conditions. In
                      addition, experimental data were corrected for
                      temperature-induced system drifts by simulating the
                      electrical circuit of the sensor system using spectral
                      measurements [...]},
      keywords     = {Dissertation (GND)},
      cin          = {ZEA-2},
      cid          = {I:(DE-Juel1)ZEA-2-20090406},
      pnm          = {899 - ohne Topic (POF3-899)},
      pid          = {G:(DE-HGF)POF3-899},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/190118},
}